Introduction to Ecological Risk Assessment - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 586

the bacterial species Sporosarcina pasteurii. Since these crystals form sticking
wedges between the sand grains, the strength and stiffness of the sand increases
significantly.
An extended review of state of knowledge of soil Biodiversity, its functions, its
contribution to Ecosystem Services and its relevance for the sustainability of human
society is given in European Commission DG ENV ( 2010 ). In the present section,
some 'basic' Ecosystem Services will be described in more detail.
13.4.3.2 Soil Structuring
A good soil structure facilitates root penetration, the exchange of gas (oxygen, car-
bon dioxide, methane and nitrous oxides) and water with the atmosphere, and the
buffering of water. It also provides space for soil organisms. A well-structured soil
is built up of aggregates, that is, individual soil particles that are joined into clus-
ters. Such a soil is therefore relatively insensitive to crusting and clodding and, in
the case of clay soils, to becoming sticky when wet. Well-aggregated soils are ero-
sion resistant, since the aggregates are heavier than the individual soil particles. In
dispersed soils the relatively light soil particles are sensitive to being blown away
by wind or washed away by water. Moreover, the drainage is better in aggregated
soils. Aggregates facilitate drainage, while having a high water storage capacity at
the same time. Boyle et al. ( 1989 ) showed that untilled soil has more than three
times the water infiltration capacity of mouldboard-ploughed soil. An aggregated
soil facilitates easy seedling emergence and deep, prolific plant-root systems. Soil
structuring indirectly influences heat and cold storage and, hence, even climate.
Many organisms contribute significantly to the loosening up of the soil. With
regard to sustainable agricultural practices, reference is often made to the blessings
of the soil ecosystem as a natural tillage performer. These blessings relate to good
moisture conservation, high water infiltration, limited water run-off and the long-
term building up of organic matter. Soils that are not burdened by the footsteps of
humans and larger animals, traffic, agricultural machinery, etc., have a loose, slightly
puffed-up structure.
Several glue-like substances such as gums and waxes, are produced by fungi and
bacteria that are responsible for 'water-stable' aggregates, that is, aggregates that
are stable when wet. Also the threads produced by fungi contribute to aggregate
formation. Fungal hyphae, for example, physically bind soil particles together, cre-
ating stable aggregates that help increase water infiltration and soil water-holding
capacity (Ingham 2000b ; Van der Wal 2007 ).
Several soil-dwelling organisms are crucial for structuring the soil, for example,
earthworms by digging tunnels (macro pores). Earthworm burrows enhance water
infiltration. Earthworm tunnelling can result is an increase in water absorption by
a factor of four to ten (Edwards and Bohlen 1996 ). Their tunnels, often lined with
nutrient-rich organic compounds, can remain in place for years. Moreover, earth-
worms form clusters of soil material ( worm casts ), ranging in diameter from one or
two millimetres to one centimetre, as the result of their excretions from the diges-
tive tract after consumption of soil particles and microbes. A good population of